T cells play a central role in immunity to viruses but our knowledge of what is required at the cellular and molecular level to generate an anti-viral T cell response is not as extensive as it needs to be, particularly in light of the emergence of new strains of virus, and the potential of viruses to be use for bioterrorist activities. We propose to study several novel molecules in a superfamily, the TNF/TNFR family, that might be essential to the development of an anti-viral T cell response. These molecules might be targets for subverting the immune system away from protection either by the virus itself or in worse case scenario by bioterrorist manipulation. Conversely these molecules might be targets for improving our ability to effectively vaccinate. In all cases, without a basic understanding of their potential importance to the generation and persistence of anti-viral T cells, we are at a disadvantage. We hypothesize that the interactions of OX40/OX40L, HVEM- LIGHT, and 4-1BB-4-1BBL will play essential non-redundant roles in directing the priming of CD8 and CD4 cells induced by vaccinia virus and for inducing long-lasting memory. We will use both wt and recombinant vaccinia containing a defined CDS T cell epitope of ovalbumin in experiments with knockout animals for LIGHT, OX40, and 4-1BB to show their contributions to generating vaccinia-specific T cells. Additionally, we will use TCR transgenic CDS and CD4 T cells, reactive with OVA, and deficient in one of the above molecules, to specifically define the role on a T cell by tracking adoptively transferred T cells in vivo. These experiments will provide invaluable data on the importance of costimulatory members of the TNFR/TNF family in controlling T cell responses to vaccinia and highlight molecules that might be targets for future vaccination strategies.